1. Field of the Invention
The present invention relates to a top nozzle for a nuclear fuel assembly having a spring insert hole.
2. Description of the Related Art
As is well known to those skilled in the art, a nuclear reactor is a device in which a fission chain reaction of fissionable materials is controlled for the purpose of generating heat, producing radioactive isotopes and plutonium, or forming a radiation field.
Generally, in light-water reactor nuclear power plants, enriched uranium, which is increased in the ratio of uranium-235 to 2% through 5%, is used. To process enriched uranium into nuclear fuel to be used in nuclear reactors, a forming process, by which uranium is formed into a cylindrical pellet having a weight of about 5 g, is conducted. Several hundreds of pellets are retained into a bundle and inserted into a zirconium tube under vacuum conditions. A spring and helium gas are supplied into the tube and a cover is welded and sealed onto the tube, thus completing a fuel rod. A plurality of fuel rods constitutes a nuclear fuel assembly and is burned in a nuclear reactor by nuclear reaction.
FIG. 1 is a front view showing a typical nuclear fuel assembly. FIG. 2 is a perspective view of a top nozzle 30 provided with spring clamps 31 having spring insert holes 31a formed by milling, according to a conventional technique (U.S. Pat. No. 5,213,757).
As shown in FIG. 1, the nuclear fuel assembly includes a plurality of support grids 10 through which fuel rods (not shown) are inserted, and a plurality of guide thimbles 20 which are coupled to the support grids 10. The nuclear fuel assembly further includes a top nozzle 30 which is coupled to the upper ends of the guide thimbles 20, a bottom nozzle 16 which is coupled to the lower ends of the guide thimbles 20, and the fuel rods (not shown) which are supported by springs and dimples which are formed in the support grids 10.
As shown in FIG. 2, the top nozzle 30 includes fastening parts 15, spring clamps 31 and hold-down spring units 32. The fastening parts 15 function to couple the top nozzle 30 to alignment pins in an upper core plate. Each spring clamp 31 has the spring insert holes 31a formed therein. The end of each hold-down spring unit 32 is inserted into a corresponding spring insert hole 31a. Fastening pin holes 33′ are formed through the upper surface of each spring clamp 31 above the corresponding spring insert holes 31a. T-slots 14 are formed in each spring clamp 31 and respectively communicate with the fastening pin holes 33′. The hold-down spring units 32 are inserted into the corresponding spring insert holes 31a and fastened to corresponding spring clamps 31. Each spring insert hole 31a is formed by milling in such a way to insert a milling tip (not shown) into the T-slot 14 formed in the upper surface of the spring clamp 31.
Each hold-down spring unit 32 includes a first spring 32a having a first neck part 32a′, and a second spring 32b and a third spring 32c which are coupled to the first neck part 32a′. The hold-down spring unit 32 is configured such that the first, second and third springs 32a, 32b and 32c are stacked on top of one another. To couple the hold-down spring unit 32 to the top nozzle 30, a spring junction end of the hold-down spring unit 32, which is opposite to the first neck part 32a′, is inserted into the corresponding spring insert hole 31a in a horizontal direction. Thereafter, a fastening pin 33 is inserted both into the corresponding fastening pin hole 33′ of the spring clamp 31 and a fastening pin hole 32a″ of the hold-down spring unit 32 in the vertical direction. Thereby, the hold-down spring unit 32 is coupled to the top nozzle 30. Here, to prevent the fastening pin 33 from being removed, the upper end of the fastening pin 33 is fastened to the spring clamp 31 by spot welding. In FIG. 2, reference numeral 40 denotes an upper plate, and reference numeral 41 denotes an upper plate slot.
As shown in FIG. 1, the top nozzle 30 having the above-mentioned construction is assembled with the elements of the nuclear fuel assembly. Subsequently, as is well known, the nuclear fuel assembly is installed in a core and disposed between an upper core plate (not shown) and a lower core plate such that the hold-down spring units 32 are supported by the lower surface of the upper core plate. As such, the nuclear fuel assembly is installed in the core of the nuclear reactor in which nuclear fission is caused and is used as fuel for nuclear power generation.
When the nuclear fuel assembly, which is installed in the nuclear reactor, is used as nuclear fuel, the hold-down spring units 32 of the top nozzle 30 conduct a shock absorption function against vibrations generated by a hydraulic uplift force induced by the flow of coolant during the operation of the nuclear reactor, thermal expansion attributable to an increase in temperature, irradiation growth of the nuclear fuel tube due to neutron irradiation for a long period of time, or axial length variation owing to creep. Thereby, a mechanical-structural stability of the nuclear fuel assembly is ensured.
However, in the top nozzle 30 according to the conventional technique, when the uplift force is applied to the T-slot 14 provided for forming each spring insert hole 31a by the end of the hold-down spring unit 32 inserted into the spring insert hole 31a, the T-slot 14 widens, causing the hold-down spring unit 32 fastened to the spring clamp 31 to become loosened. Thereby, a force supporting the nuclear fuel assembly is markedly deteriorated.
Furthermore, in a case where the spring insert holes 31a are formed in the top nozzle 30 by milling in the direction in which the hold-down spring units 32 are inserted into the spring insert holes 31a without forming the T-slots 14, the milling operation is impeded by the fastening parts 15 which protrude from the upper surface of the top nozzle 30 at positions opposite to the spring clamps 31. Thus, it is very difficult to precisely machine each spring clamp 31 such that the interior of the spring insert hole 31a has a shape corresponding to the end of the hold-down spring unit 32.
Thereby, the end of the hold-down spring unit 32 cannot be brought into close contact with the inner surface of the spring insert hole 31a. As a result, the hold-down spring units 32 cannot reliably absorb vibrations of the nuclear fuel assembly when the unclear reactor is operated.
Moreover, in an extreme case, the above-mentioned problems in the conventional technique may cause a deformation or breakage of the nuclear fuel assembly.